Problems with early models

It turns out however that there are serious problems with this simple planetary model of the atom. These relate to the fact that the electron, in orbiting the nucleus, undergoes an acceleration (recall that an acceleration arises from a change in velocity, which can be a change in speed and/or a change in the direction of motion). Maxwell's theory of electromagnetism then predicts that any charge which is accelerated will emit electromagnetic radiation; for example, X-ray machines produce X-rays by accelerating electrons through fairly high potential differences. Thus, in this model we would expect the electron to be continuously emitting radiation, and as it does it would lose energy, and eventually collapse into the nucleus.

At first these were thought to be good features of the model, as it was known that atoms do emit light when heated, and also that many atoms, particularly the heavier ones, do have a finite lifetime. However, quantitatively the predictions are completely wrong. The light that comes from atoms of a given element occurs only with a certain set of discrete frequencies, whereas the light emitted in this model contains a wide range of frequencies. Furthermore, the time of collapse of any atom in this model turns out to be of the order of 10^-9 s, which is much too short compared to the lifetimes of, in particular, the lighter atoms, which are relatively stable.